The invention described herein relates generally to methods for applying coatings to substrates, and more particularly to methods for applying regulated composite coatings, of uniform or variable composition and configuration, onto a wide variety of solid, or even liquid, substrate surfaces, and to the unique coatings produced by these methods.
There presently exist a wide variety of methods for applying coatings to substrates. Many of these methods require the coating material to be melted or vaporized, or even ionized, while being physically restrained in some manner, as for example by being melted in a crucible or vaporized and passed through a nozzle. For some chemically reactive materials, or materials with very high melting temperatures, crucibles and nozzles are not practically available. However, even when available, they often act as a contaminant source for the resulting coating, and further, being costly and short lived, greatly add to the expense of the coating operation.
In the well known fundamental technique of coating by vapor deposition, the coating material is melted and evaporated, with a coating formed by the subsequent condensation of the vapor upon a substrate. There are many known variations to this basic technique. For example, Blum et al in U.S. Pat. No. 4,451,503 issued May 29, 1984, teaches a process where metal carbonyl is heated in a reaction chamber to produce metal compound vapors that are photodecomposed by ultraviolet radiation of wavelengths less than 200 nm. The released metal atoms are condensed onto a substrate. Matsuda et al in U.S. Pat. No. 4,569,855 issued Feb. 11, 1986, discloses a process for forming a deposition film, containing silicon atoms, on a substrate, by exciting and decomposing a gaseous silicon compound with light energy. Tanaka et al in U.S. Pat. No. 4,604,294 issued Aug. 5, 1986, teaches the vacuum vapor deposition of an organic compound. The compound is vaporized by a laser beam having an energy level corresponding to that of the chemical bond of the organic compound. Swain in U.S. Pat. No. 4,427,723 issued Jan. 24, 1984, discloses a method and apparatus for vacuum deposition and annealing, wherein a coating material is evaporated by a first laser beam, while a second laser beam locally heats the substrate to promote annealing and contaminant elimination of the resulting coating.
In a related area of coating technology, there presently exist various ion sources that may be used, for example, to implant ions in assorted materials. For instance, Umemura et al in U.S. Pat. No. 4,624,833 issued Nov. 25, 1986, teaches a liquid metal ion source wherein a source material is melted, by resistance heating or by a laser beam, and held in a reservoir. The melted source material is fed from the reservoir to a separated emitter, from which ions are ejected under a high electric field. It is pointed out that the source material must be properly selected so that it does not react violently with the emitter, and thus stop the ion extraction process. Because of this consideration, source materials are limited to various copper alloys.
Particle implantation is also taught by Bruel et al in U.S. Pat. No. 4,585,945 issued Apr. 29, 1986. In this teaching a beam of high energy particles are shot into a cloud of secondary particles, to impart energy to the secondary particles, and thus enable them to penetrate a metal substrate.
Once particles or ion beams have been produced, there presently exist many known processes for their subsequent manipulation. For example, Ashkin et al in U.S. Pat. No. 4,092,535 issued May 30, 1978 disclose an improved optical device for the levitation of a particle in vacuum; and Dalglish in U.S. Pat. No. 4,464,573 issued Aug. 7, 1984, teaches a charged particle beam focusing device wherein a pair of rod-like electrodes, of different electrical potential, are arranged on either side of the focused beam.
An apparatus and a method for depositing ionized clusters, consisting of 100 to 1000 atoms, on a substrate are disclosed by Takagi in U.S. Pat. No. 4,152,478 issued May 1, 1979, and in U.S. Pat. No. 4,217,855 issued Aug. 19, 1980. The material to be deposited is heated within a sealed crucible having one or more nozzles. The vapor of the material is ejected through the nozzles and into a low pressure region that surrounds the crucible. The resulting adiabatic expansion forces the vapor into a supercooled state that results in the formation of vapor aggregates or clusters. The clusters are then ionized by electron bombardment, and provided with kinetic energy by an extractor electrode that is held at a negative potential.
In other technology, first described by Zeleny in Phys. Rev. 10, 1 (1917), it is well known that an electric field applied to the surface of a liquid can distort the fluid surface and cause the ejection of charged droplets or even, as later discovered, multiatomic particles or single atoms. This effect is caused by the electric field producing forces which interact on the material additionally with the gravity forces, internal molecular and atomic binding forces, and surface tension forces, that are normally present in any liquid. These forces all interact to form a dynamic pressure distribution on the surface of the liquid that is functional of the local radius of curvature of the fluid surface. Small quantities of the liquid are forced to flow into regions within which the local radius of curvature is impelled to rapidly diminish, and are ejected from the fluid surface as droplets. It is very difficult, if not impossible, to precisely control the size distribution of the particles produced by this method. .he technique is very well-known and, to name a few of its applications, has been used in printing, ion etching, pattern deposition and ion implantation.
For example, Hendricks in U.S. Pat. No. 3,582,958 issued June 1, 1971 discloses a printer having an ion beam source in which a beam of large or small, single or multiatomic ions are extracted from a liquid surface by an electric field. The beam is directed upon a web, to impress a message upon the web.
The early activity in this art of particle production was all carried out using media that are liquid at normal room temperature, such as water, ink, glycerol and mercury. In later applications, however, as exemplified by the teaching of Mahoney et al in U.S. Pat. No. 4,264,641 issued Apr. 28, 1981, metal droplets have been formed by the application of an intense electric field to the liquid surface of a molten metal. In Mahoney et al, the metal is melted by a crucible heater and placed in a refractory reservoir, from which it is drawn into a refractory nozzle that terminates in a short capillary tip. when the molten metal approaches the nozzle tip, it enters a region of intense electric field established by the application of high positive voltage to the nozzle. The electric field is maintained between the positive nozzle and an extractor electrode that is held at a negative potential. Both the molten metal surface and the nozzle tip are bombarded by backstreaming electrons emitted from the extractor electrode. Additional backstreaming electrons can be emitted from the extracted metal droplets themselves, or from small plasmas formed from the droplets. Backstreaming electrons heat the molten metal surface and create a runaway condition wherein the heating process is not under control. However, in some applications, negatively biased thermionic electron emitters have been specifically used to provide electrons to heat the metal surface and keep it molten during high electric field particle extraction. Nevertheless, where uncontrolled backstreaming electrons are present, as is often the case, they can heat and melt the nozzle tip itself, even though it be made of a refractory ceramic or some other high-melting-point material such as stainless steel. When this happens, the material of the nozzle tip intermixes with and contaminates the metal droplets which the process is seeking to produce.
In my U.S. patent application Ser. No. 911,847 filed Sept. 26, 1986, issued Sept. 27, 1988 as U.S. Pat. No. 4,774,037 and titled "Method For Producing Solid or Hollow Spherical Particles Of Chosen Chemical Composition and of Uniform Size," I provide a method for producing large quantities of high melting temperature solid or hollow spherical particles of a predetermined chemical composition and having a uniform and controlled size distribution. An end of a solid or hollow rod of the material is rendered molten by a laser beam. Because of this, there is no possibility of the molten rod material becoming contaminated with extraneous material. In various aspects of the invention, an electric field is applied to the molten rod end, and/or the molten rod end is vibrated. In a further aspect of the invention, a high-frequency component is added to the electric field applied to the molten end of the rod. By controlling the internal pressure of the rod (when the rod is hollow), the rate at which the rod is introduced into the laser beam, the environment of the process, the vibration amplitude and frequency of the molten rod end, the electric field intensity applied to the molten rod end, and the frequency and intensity of the component added to the electric field, the uniformity and size distribution of the solid or hollow spherical particles produced by the inventive method are controlled. The polarity of the electric field applied to the molten rod end can be chosen to eliminate backstreaming electrons, which tend to produce run-away heating in the rod, from the process.
It is, thus, apparent that the presently existing methods for applying coatings to substrates are unsatisfactory in regard to providing pure coatings of materials having a very high melting temperature. The prior art is also wanting in regard to providing a methodology for the production of coatings created from charged clusters of atoms. It generally appears that the prior art of coating substrates is seriously inadequate in that regulated multi-material composite coatings of variable composition and configuration frequently cannot be provided. The prior art is, further, apparently lacking with regard to the deposition of coatings onto liquid surfaces.